Bragg gratings are structures with a periodic variation in refractive index that are usually formed in optical components such as holograms, waveguides, and optical fibers. These structures reflect an incident light beam such that the reflected light has a narrow spectral bandwidth with a central wavelength. Wavelengths outside of the reflected bandwidth are transmitted without alteration.
The fraction of incident light that is reflected is determined by the magnitude of the refractive index variation and by the number of refractive index periods provided in the structure. Reflectivity greater than 99% can be obtained in Bragg structures that are only 100 μm to 300 μm thick and have refractive index changes near 0.01. This combination of high reflectivity over a narrow spectral bandwidth has several interesting applications. Bragg gratings, for example, are used in optical communication as stabilizers for pump lasers, narrowband wave division multiplexing (VDM) add/drop filters, and gain-flattening filters. Additional applications include narrow-band filters for laser protection, Raman spectroscopy, wireless optical communication, and light detection and ranging (LIDAR).
Many of these applications demand that the grating operate over a relatively wide temperature range without significant change either to its reflectivity or to its central, or Bragg, reflected wavelength. Unfortunately thermal expansion and contraction of the Bragg grating material alters the performance of the grating.